Industrial furnaces, in particular vacuum furnaces, are very often employed to "heat treat" metals. The "heat treating" is involved, for instance, in brazing one metal piece to another. In an another example "heat treating" is involved in hardening a metal by heating a workpiece to a specified temperature, holding the workpiece at that temperature for a specified period of time (which causes the microstructure of the workpiece to change to a desired arrangement), and then "freezing" the microstructure in the desired configuration. The so called "freezing" is accomplished by quenching the workpiece. In many applications, the more rapidly a workpiece is quenched, the better is the desired microstructure. A common example of such hardening would be the heat treatment of tool and die steels, such as ASI grades A, D and H as well as others. In prior art vacuum furnace arrangements, workpieces have been heated in a hot zone, in response to radiation of heat from heating elements located within the vacuum furnace. More particularly, the heating elements are located in the vacuum furnace within a hot zone. Hot zones are very often defined by a chamber made up of heat insulating material, although other arrangements for defining the hot zone are known. The heating elements are energized by electrical energy and as they get progressively hotter they radiate heat, which radiation heat is transmitted across the vacuum condition in the furnace to a workpiece location to heat the workpiece, or workpieces. The foregoing technology has been successful but it does require time. At low radiation temperatures (e.g. 1200.degree. F.) the response time to heat the workpiece is slow. The present arrangement gets the heating process underway more quickly than radiation per se because convection heating is employed at lower temperatures. In the present arrangement, the vacuum chamber is initially pumped down to a vacuum condition. Thereafter, the vacuum furnace is backfilled with an inert gas, such as nitrogen, to near atmospheric pressure. The inert gas is circulated in close proximity to the heating elements, as they heat up. Before the heating elements are in the heated condition to heat a workpiece by radiation heat, they do in fact start heating the inert gas. During the early heating process, the heated inert gas is passed over the workpieces and transfers heat thereto by convection. This early heating step saves time in the overall cycle and saves energy by getting the workpiece to the desired temperature more quickly than by the radiation method per se. When the workpiece has been heated to the desired temperature, which is measured by a thermocouple, or some other device, the workpiece is held at that temperature for a specified amount of time, i.e. until the microstructure of the workpiece is transformed to a desired microstructure.
As was mentioned above, the workpiece must be quenched to "set" the desired microstructure. Heretofore, the method for quenching has been to force a large amount of inert gas into the hot zone from a number of different angles to create turbulence in the hot zone. By creating turbulence in the hot zone, the workpiece comes in contact with many surfaces of many different layers of gas. The layers of gas pass over, and in contact, with the workpiece and in so doing heat is transferred to the quench gas from the workpiece. In short, exposure of the workpiece to multiple surfaces of the quench gas cool the workpiece rapidly. In prior art quenching arrangements, the workpiece has not been cooled as uniformly as desired because of the temperature non-uniformity of the quench gas surfaces passing over the workpiece. The present system improves the quench time and the quench uniformity, by providing many cooled surfaces of inert gas, to be applied to the workpiece. In a preferred embodiment of the present arrangement, the system is structured to divide the plenum (which surrounds the hot zone) into upper and lower plenum chambers, at least first and second plenum chambers. When the quenching step commences, a valve connecting the vacuum furnace housing chamber to one or more surge tanks is opened and inert gas, which is under a relativity high pressure in the surge tanks, surges into the first plenum chamber. The surging "quench gas" rapidly passes into the hot zone to come into contact with the workpiece. Simultaneously therewith, a recirculating turbine fan device draws the inert gas from the hot zone and directs it to the second plenum chamber. Of course, the gas which is being drawn from the hot zone has been in contact with the workpiece and has taken some of the heat from the workpiece and is therefore, itself, heated up. In drawing the previously heated gas from the hot zone, to be loaded into the second plenum chamber, the previously heated gas is directed through a pair of heat exchangers which act to withdraw heat from the previously heated gas. The gas is then passed through the second plenum chamber into the hot zone to come into contact with the workpiece and thereafter is drawn from the hot zone in response to the operation of the recirculating turbine fan. The quenching gas is then passed, alternately, between the first and second plenum chambers, by way of the hot zone and the heat exchangers. Hence the quenching gas repeatedly comes in contact with the workpiece and is cooled by passing through heat exchangers. These repeated cycles provide many relatively cool surfaces of gas layers to the workpiece and have been found to reduce the general quenching time as well as improve the uniformity of the quench of the workpiece.